Conventionally, 1,3,5-trioxane is obtained by the cyclization reaction of formaldehyde in the presence of an acid catalyst or a solid acid catalyst. A 1,3,5-trioxane-containing vapor obtained by the cyclization reaction is supplied from a reactor to a distillation tower. The 1,3,5-trioxane-containing vapor boiling in the distillation tower is concentrated and discharged, and then this concentrated 1,3,5-trioxane-containing vapor is extracted with a water-insoluble organic solvent. Further, the 1,3,5-trioxane-containing vapor boiling in the distillation tower may be directly extracted with a water-insoluble organic solvent. In both cases, 1,3,5-trioxane is extracted by both a distillation tower and an extraction tower, and an extracted liquid is converted into a solution containing a small amount of 1,3,5-trioxane by a fractionator, and then the solution is refluxed into the extraction tower.
For example, in the case of JP 1982-200383, a distillation tower and an extraction tower are separately provided in order to prepare 1,3,5-trioxane. 1,3,5-trioxane concentrated at the top of the distillation tower is introduced into the extraction tower, and is then extracted with a water-insoluble organic solvent. This method of preparing 1,3,5-trioxane disclosed in JP 1982-200383 is problematic in that an equipment investment cost excessively increases because a distillation tower and an extraction tower are separately provided in order to prepare 1,3,5-trioxane, in that the preparation efficiency of 1,3,5-trioxane remarkably decreases because a large amount of formaldehyde and 1,3,5-trioxane, other than 1,3,5-trioxane extracted with extractant in the extraction tower, is discharged to the outside of a system, and in that distillation loss increases in the recovery process for recovering and reusing 1,3,5-trioxane.